By virtue of the broadcast property of wireless transmission media, all wireless terminals within the propagation scope might receive information from a source node. As a result, information destined to a target node from the source node might be eavesdropped by other nodes, secrecy is reduced and information cannot be transmitted securely. Thus, the secrecy of wireless communication systems becomes a focus of attention. Existing information security research mainly focuses on physical layer upper layer confidentiality agreements, i.e. ensuring information transmission security by confidentiality agreements after encapsulating physical layer transmission data.
At present researchers place emphasis on how to increase the secrecy capacity of wireless communication systems without upper layer encryption, i.e., how to make eavesdropping nodes unable to obtain or obtain as little as possible information destined from a source node to a destination node. Researchers refer to the difference between a capacity from a source node S to a destination node D and a capacity from the source node S to an eavesdropping node E as a secrecy capacity.
Meanwhile, in a traditional cellular network, if a destination node is not located at some areas where communication with a source node is impossible, such as cell edges or areas where source node signals cannot be received because of tall building obstructions, communication between the destination node and the source node is impossible, or signals therebetween are weak and the dropped-call rate increases. If, on the basis of original facilities, some relay nodes are added between the source node and the destination node or part of source nodes/destination nodes act as relay node to forward information, then the link quality between the source node and the destination node can be improved and further the system performance can be enhanced. Therefore, the research on relay technology is of great significance to new-generation wireless communication systems.
In the relay technology research, relaying may be divided into non-reproduction relaying and reproduction relaying according to different forwarded information. Non-reproduction relaying is to simply amplify received information and send it to a user, while reproduction relaying is that a relay station decodes received information and send it to a user after recoding it.
In view of advantages of relay technology, to combine physical layer information security with relaying now becomes a new research focus in the wireless field. It is found in research when a relay node approaches a destination node, the capacity from a source node to the destination node is greater than the capacity of eavesdropping links. Therefore, in cooperation relay systems with secrecy capacity restrictions, relay selection is an effective method to enhance the system performance and reduce the implementation complexity. At the same time, people find when other node assists the source node in sending interference, i.e. both the destination node and the eavesdropping node receive interference information and the destination node already learns the interference information, the secrecy capacity of systems can be enhanced effectively.
When there exist multiple relay nodes between the source node and the destination node, existing efforts to combine physical layer information security with relaying mainly concentrate on a reproduction relaying system: in the first phase, the source node broadcasts information to the multiple relay nodes, and the system selects the optimal node from successfully decoded relay nodes for forwarding and then selects an interfering node from other relay nodes for sending interference; in the second phase, the selected node is decoded and forwards information to the destination node after being recoded, and the interfering node sends interference information. The eavesdropping node can receive information sent from the relay node and the interfering node in the second phase. Nevertheless, the reproduction relay mode means high complexity to the system implementation in the respect of the system's real work.
Apparently, when ensuring the system secrecy capacity nowadays, the system complexity is high during a relay transmission in the reproduction relay mode. Therefore, there is a need for a non-reproduction relay mode based technical solution to control a relay transmission, which not only ensures the system secrecy capacity but also is easy for implementation with a simple system structure.